1. Field of the Invention
The present invention relates to an ink set comprising at least two ink compositions, an inkjet recording apparatus comprising the ink set, and a method of forming an image using the ink set, and more particularly, to an ink set which reduces bleeding between ink compositions having different colors from each other, especially a black ink composition and a color ink composition without detriment to an image density on a recording medium, an inkjet recording apparatus including the ink set, and a method of forming an image using the ink set.
2. Description of the Related Art
Inkjet recording apparatuses form an image on a recording medium by discharging ink droplets through fine nozzles. Inkjet ink should have an excellent drying property on a recording medium, and bleeding of an image formed therefrom must be reduced. Further, inkjet ink should exhibit a uniform image irrespective of the types of recording medium, and when printing colors, bleeding must be reduced at the interfaces between colors. In addition, inkjet ink should have excellent image durability, such as water resistance, light resistance, abrasion resistance, and the like.
Various ink compositions have been proposed to satisfy the above requirements. In particular, a method of preventing bleeding between colors by increasing a drying speed of ink has been proposed. Drying of the ink occurs through two mechanisms—evaporation and penetration. Evaporation depends on a vapor pressure of the ink, while penetration depends on interfacial energy between the ink and a recording medium. To shorten a drying time by increasing a speed at which ink penetrates into a recording medium and decrease bleeding between colors, many methods, for example, addition of penetrants, have been proposed. For example, U.S. Pat. No. 5,156,675 discloses the addition of diethylene glycol monobutyl ether into ink, U.S. Pat. No. 5,196,056 discloses the addition of diethylene glycol monobutyl ether and sulfonyl 465 into ink, U.S. Pat. No. 6,132,502 discloses the addition of glycol monobutyl ether, such as diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, propylene glycol monobutyl ether, and dipropylene glycol monobutyl ether into ink.
The penetrants described above increase a drying speed of the ink, thus reducing bleeding between colors. However, if a large amount of penetrant is present in the ink composition, an insufficient amount of ink remains on a surface of a recording medium, and thus an image density may be decreased. Especially, when realizing a black image by mixing Cyan/Magenta/Yellow to realize a mixed black image, the image density may be increased using color matching with software, even though each image density of the color inks is low. However, in this case, consumption of the inks is increased. Thus, it is advantageous to use black ink to print the black image.
Meanwhile, in general, a drying time of the black ink is longer than a drying time of the color ink. When the drying time of the black ink is significantly longer than that of the color ink, the bleeding cannot be sufficiently reduced even when the color inks are rapidly dried. Considering that one of the goals is to have inkjet printing speed of 30 ppm, 60 ppm or faster for a text mode, the drying time of the black ink is required to be reduced in view of the bleeding and the printing speed. As described above, when a penetrant is added to ink to reduce the drying time, the image density must remain high. In the conventional method, the amount of the penetrant was determined without considering the color of the ink. It is important to add a sufficient amount of penetrant depending on the color of the ink.